Wind power installations, like wind turbines or wind power plants, have to comply with given and specified grid codes, which are ruling the behavior of the wind power installations for given situations. These grid codes are specified in dependency of the capabilities of a given grid in a given country. Thus the grid codes known might differ from country to country.
The grid frequency is scheduled and fixed for a given electrical grid. The frequency is influenced by the amount of active power, being fed into the grid by power installations, and by the amount of active power, being consumed and thus taken out of the grid. A shift of this balance results in a grid frequency change. A grid frequency change, which is outside a given tolerance band, is supposed to be a grid fault.
Given grid codes stipulate for this case, that wind power installations have to reduce the feed-in of active power to counteract a rise in the grid frequency or that wind power installations have to increase the feed-in of active power to counteract a fall in the grid frequency.
The grid voltage, which is scheduled and fixed for a given electrical grid, is influenced by reactive power. The reactive power is given and influenced by components of power installations and their technical behavior and of consumers, which are connected to the grid as well.
A grid voltage change, which is below a given tolerance, is supposed to be a grid fault. Given grid codes stipulate for this case, that wind power installations have to increase at least reactive power to counteract a fall in the grid voltage.
Thus the grid codes address the behavior of wind power installations in case of specified grid faults. Respective grid fault functionalities are known as “Frequency Ride Through, FRT” or as “Low Voltage Ride Through, LVRT” for example.
The grid codes aim for a specified and known behavior of the wind power installations during grid faults. If possible they should stay connected with the electrical grid instead of disconnecting thus a faster grid fault response of the wind power installations in view to the stability of the electrical grid is ensured.
If a wind turbine is exposed to the consequences of a grid fault, the load situation, which is acting on the rotating parts of a wind turbine (rotor, hub, shaft, generator) is decreased. Thus results in higher speed and in higher vibrations as the load, acting on the components, is changed. This rotational kinetic energy is present in the wind power installation during the grid fault and needs to be handled in a specific way to avoid harming of components.
Load changes will be experienced most at the rotating blades of the wind turbine, if the wind turbine is a so called “direct drive” wind turbine (thus there is no gear box connected with the generator). The load changes will result in a kind of “whiplash”, which put high strains on the blades and thus causing excessive wear and damages.
It is known to use resistive loads in a converter of a wind power installation to reduce the electrical power, which is present in the wind power installation during the fault.
Thus redundant power will not damage the wind turbine converter or other components.
It is even known to use so called “dumpload” resistors in the wind power installation. They are used to convert redundant power into heat. Due to the function of the resistors they show a considerable size, weight and price.